1. Field
The present disclosure relates to nuclear reactors and, more specifically, fuel supports for supporting fuel assemblies and associated methods for modifying coolant flow to fuel assemblies.
2. Description of Related Art
The statements in this section merely provide background information related to the present disclosure and may not constitute prior art.
A nuclear reactor pressure vessel (RPV) has a generally cylindrical shape and is closed at both ends, e.g., by a bottom head and a removable top head. A top guide is spaced above a core plate within the RPV. A core shroud, or shroud, surrounds the core plate and is supported by a shroud support structure. Particularly, the core shroud has a generally cylindrical shape and surrounds both the core plate and the top guide. The top guide includes several openings, and fuel assemblies are inserted through the openings and are supported by the core plate. The core plate includes a flat plate supported by a plurality of beams.
A nuclear reactor core includes a plurality of individual fuel assemblies that have different characteristics that affect the strategy for operation of the core. For example, a nuclear reactor core typically has several hundred individual fuel assemblies that have different characteristics, each fuel bundle having a plurality of fuel rods. The fuel assemblies are arranged within the reactor core so that the interaction between the fuel assemblies satisfies regulatory and reactor design guidelines and constraints. In addition the core arrangement determines the cycle energy, which is the amount of energy that the reactor core generates before the core needs to be refreshed with new fuel elements, the core loading arrangement preferably optimizes the core cycle energy.
A core cycle is determined from one periodic reactor core refueling to a second reactor core refueling. During the course of the cycle of operation, the excess reactivity, which defines the energy capability of the core, is controlled in a variety of ways. Specifically, a burnable poison, e.g., gadolinia, is incorporated in the fresh fuel. The quantity of initial burnable poison is determined by design constraints typically set by the utility and by the National Regulatory Commission (NRC). The burnable poison controls most, but not all, of the excess reactivity. A second way is through the manipulation of control rods within the core. Control rods control the excess reactivity. Specifically, the reactor core contains control rods which assure safe shutdown and provide the primary mechanism for controlling the maximum power peaking factor. The total number of control rods available varies with core size and geometry, and is typically between fifty and two hundred and fifty. The position of the control rods, i.e., fully inserted, fully withdrawn, or somewhere between, is based on the need to control the excess reactivity and to meet other operational constraints, such as the maximum core power peaking factor.
Coolant is introduced in the core to cool the core, to be transitioned into steam as a working fluid for energy generation, and to provide neutron source aid in the nuclear reaction. Normal coolant flow enters the fuel assemblies as a single phased flow with slightly sub-cooled coolant. The flow approaches the fuel support vertically upward and then turns horizontally as the flow enters the inlet to a fuel support supporting a fuel assembly. The flow then passes through an orifice of the fuel support to provide a pressure drop to assist coolant distribution to the fuel assemblies. The flow then turns vertical and enters the lower tie plate of the fuel assembly and is distributed around the individual fuel rods of the fuel assembly.
Known reactors have included fuel support orifice regions within the core, one around the peripheral and one near the center. The peripheral region includes all fuel locations around the periphery of the core, and the center region includes the remainder of the locations. The fuel support orifices are designed to limit the flow to the fuel assemblies in the peripheral region to about half of the flow per fuel element of the center region. Limiting the peripheral flow by this magnitude has permitted the very low power peripheral fuel elements to saturate the coolant flow, but with maintaining the exit quality and average voids that are still much lower than for the other higher power region. This uneven exit quality and average void can produce inefficient steam separation and nuclear moderation.
It is also known that the coolant flow can be adjusted through varying the design of the fuel assembly. For example, it is known that each fuel assembly can include a main coolant flow channel and inlet that has a substantial constant flow. However, the fuel assemblies can also include one or more secondary coolant flow channels that can vary to adjust the coolant flow in the particular fuel assemblies. In some cases, three types of fuel assemblies can provide three different secondary coolant flows. Each such fuel assembly can be positioned in the core to provide for a desired coolant flow. For example, three different fuel assemblies have been arranged into three or more core regions. The flow of coolant through each fuel assemblies in each region can be different from the coolant flow through the fuel assemblies in each other region based on the position of the three different fuel assemblies and the layout of the regions within the core. However, this requires the manufacture of different types of fuel assemblies and/or tie plates and the administration of those different designed of fuel assemblies. Other methods have included attaching a flow restricting device to the lower tie plate of the fuel assemblies in which flow is intended to be restricted. Such lower tie plate flow restricting devices provide one method for flow modification, but often require additional administration and manufacturing procedures for attachment of the flow restricting devices to the lower tie plates.